.\"
.Dd September 24, 2022
.Dt SNP 4
.Os
.Sh NAME
.Nm snp
.Nd tty snoop interface
.Sh SYNOPSIS
.In sys/snoop.h
.Ft int
.Fn ioctl fd SNPSTTY &dev
.Ft int
.Fn ioctl fd SNPGTTY &dev
.Ft int
.Fn ioctl fd FIONREAD &result
.Sh DESCRIPTION
.Pa /dev/snp
is a snoop device which allows users to attach to any tty
and watch activities on it.
The kernel must be compiled with
.Cd "device snp" ,
or the
.Nm
module must be loaded,
for these devices to be available.
.Pp
To associate a given
.Nm
device with a tty to be observed, open the
.Nm
device and a tty device, and then issue the
.Dv SNPSTTY
ioctl on
.Nm
device.
The argument passed to the
.Xr ioctl 2
is the address of a variable of type
.Vt int ,
holding the file descriptor of a tty device.
To detach the
.Nm
device from a tty use a pointer to a value of
\-1.
.Pp
The
.Dv SNPGTTY
ioctl returns information about the current tty attached to
the open
.Nm
device.
.Pp
The
.Dv FIONREAD
ioctl returns a positive value equal to the number of characters
in a read buffer.
Special values defined are:
.Bl -tag -width ".Dv SNP_TTYCLOSE"
.It Dv SNP_TTYCLOSE
tty not attached.
.It Dv SNP_DETACH
.Nm
device has been detached by user or tty device has been closed
and detached.
.El
.Sh SEE ALSO
.Xr pty 4 ,
.Xr kldload 8 ,
.Xr watch 8
.Sh HISTORY
The
.Nm
device first appeared in
.Fx 2.1 .
In
.Fx 8.0
the
.Nm
driver was rewritten to work with the replaced TTY subsystem.
.Sh AUTHORS
.An -nosplit
The author of the current implementation is
.An \&Ed Schouten Aq Mt ed@FreeBSD.org .
Previous versions of
.Nm
were based on code written by
.An Ugen J.S. Antsilevich Aq Mt ugen@NetVision.net.il .
